Field of the Invention
The present invention relates to the semiconductor industry, in particular, to microlithography, and more specifically to a method of static scaling of an image in holographic lithography.
Description of the Related Art
Lithography and, in particular, photolithography is a well-known technique in semiconductor and printed circuit board (PCB) manufacture for creating electrical components and circuits. Predominantly, used for this purpose is projection photolithography, which involves projecting a mask image on the surface of a substrate, which has been covered by a layer of photoresist, before exposing both a mask and a substrate to light. As a rule, the projected image is optically reduced with a factor of 5 or more and for obtaining an image of high quality and accuracy the process dictates the use of very complicated, precision, and extremely expensive optical equipment operating in short wavelengths of ultraviolet range of spectrum, e.g., with λ of 193 nm or shorter. As a rule, the light sources used in the process are lasers.
The areas of photoresist that are exposed to light react and change chemical properties of the coating layer. The photoresist is then developed in order to remove either the exposed portions of photoresist for a positive resist or the unexposed portions for a negative resist. The pattern formed in the photoresist allows further processing of the substrate, such as, but not limited to, etching, deposition, or implantation.
At the present time, the above-mentioned projection photolithography is a basic photolithography technique used for production of integrated circuits. However, further development in the field of integrated circuit associated with demand for higher degree of integration and increase in complexity of the integrated circuits put forward the goals which in case of projection lithography leads to extraordinary material and intellectual expenditures.
One of practical problems of projection lithography is complexity of image scaling in sequential operations performed with the use of set of different masks. The scales of images produced with the use of masks installed in the same position of lithograph may have deviations that occur during the manufacture of the masks, even in the same process. Such deviations may be caused also by the use of different mask manufacturing tools, etc. The same relates to the alignment marks for registering positions of the mask in the lithograph.
In practical projection lithography, including the most advance lithography with the critical dimensions of image elements of the order of tens nanometers, the above problem is solved by microadjustment of the image scale by minutely displacing the mask in direction of the optical axis (in the Z-axis direction perpendicular to the image plane). In order to satisfy precision mask scaling requirements these displacements are carried out with the use of extremely complicated and expensive equipment since such an adjustment involves the use of a set of complicated micro-interferometric devices and tools for micronanopositioning. A provision of such an equipment in combination with the basic optical components of the system is a decisive factor in determining the manufacturing cost of the final product.
On the other hand, known in the art is so-called holographic lithography which to some extent is free of problems associated with the projection lithography. In holographic lithography the image formation process on a semiconductor substrate is carried out by restoring an image recorded on a special holographic mask with the use of a highly coherent laser light. The holographic mask per se is prepared by using special calculation procedures and is physically manufactured by technology which is close to the manufacture of mask in accordance with conventional projection lithography. It should be noted that the topology of the images of the holographic masks does not have anything in common with the mask topology used in the projection lithography where the mask image is similar to the original image. On the other hand, the use of the holographic masks provides significant advantages as compared to the masks of the projection lithography, i.e., the holographic masks may have several hundred longer lifetime than the masks of the projection lithography. Furthermore, defects of mask images in holographic mask practically do not affect the accuracy and quality of the target image. Another essential advantage of the holographic masks is simplicity of manufacture and low production cost. Furthermore, the topology of the holographic masks makes it possible to reproduce some image elements unattainable with the projection lithography technique.
However, until recent time the holographic lithography did not find practical application because methods for calculation of mask pattern suitable for practical application, even for masks of an average complexity, required so much time that it was practically impossible for actual application.
Nevertheless, attempts to solve the above problem of holographic lithography have been undertaken and even successfully accomplished as disclosed in the U.S. patent application Ser. No. 14/142,776 filed on Dec. 28, 2013 by Vadim Rakhovsky, et al. This application describes a method of manufacturing a holographic mask capable of producing an image pattern that contains elements of a subwavelength size along with decreased deviations from the original pattern. The original pattern is converted into a virtual electromagnetic field and is divided into a set of virtual cells with certain amplitudes and phases, which are mathematically processed for obtaining the virtual digital hologram. The calculation of the latter is based on parameters of the restoration wave, which is used to produce the image pattern from the mask, and on computer optimization by variation of amplitudes and phases of the set of virtual cells and/or parameters of the virtual digital hologram for reaching a satisfactory matching between the produced image pattern and the original pattern. The obtained virtual digital hologram provides physical parameters of the actual digital hologram that is to be manufactured.